Position responsive apparatus including a capacitive potentiometer



Oct. 17, 1967 c. F. WOLFENDALE 3,348,133

POSITION RESPONSIVE'APPARATUS' INCLUDING A CAPACITIVE POTENTIOMETEROriginal Filed Oct. 25, 1962 v 2 Sheets-Sheet 1 RELA 71 var May/N6MEMBERS ajFF/so I J 5 F :L 44 |A E I 40 \b 46 I H I I w PEF I I 8" 42 (II OH AMPLIFIER 52 H6 q M SERVO M070}? COMP ,5/ VOLTAGE OUTPUT C OMPARI?70/? MID/CA 70R 60 7o\ 60 7o 60 7o\ I" 'I F I I I I I I I I.

INVENTOQ can; B FREDE mc K WOLFENDPILE.

N am and AA ATTORNEY oct. 17, 1967 WQLFENDALE 3,348,133

POSITION RESPONSIVE APPARATUS INCLUDING A CAPACITIVE POTENTIOMETER VOriginal Filed Oct. 25, 1962 2 Sheets-Sheet z w Mas. I

- Haj/Z I v INvENTOQ A'IIQRMEQL United States Patent 3,348,133 POSITIONRESPONSIVE APPARATUS INCLUDING A CAPACITIVE POTENTIOMETER CalebFrederick Wolfendale, Great Brick Hill, near Bletchley, England,assignor to Sogenique (Electronics) Limited Original application Oct.25, 1962, Ser. No. 232,949. Di-

vided and this application Oct. 18, 1965, Ser. No. 496,933 Claimspriority, application Great Britain, Oct. 27, 1961, 38,527/ 61 4 Claims.(Cl. 323-93) ABSTRACT OF THE DISCLOSURE The capacitive potentiometerdescribed exemplarily includes a row of stator electrodes and a movablepick-off electrode capacitively coupled to the stator electrodes.successively greater alternating potentials are applied to the statorelectrodes so that the potential of the pick-ofi electrode is a smoothfunction of its position. Irregularities are reduced or eliminated byshaping the pick-off electrode or the stator'electrodes.

This invention relates to position-responsive apparatus and toc'apacitative potentiometers for use in such apparatus.

The present application is a divisional application based upon-my priorapplication Ser. No. 232,949 filed Oct. 25, 1962 and now abandoned.

More particularly the invention relates to an apparatus in which thepotentiometer comprises a series of elements arranged in line, the linebeing straight or curved as may be appropriate, elements of the seriesare energized from asuitable source of alternating potential so as toestablish an electric field along the line of elements. A pick-off orinterpolating electrode is arranged adjacent and movable relative to theelements of the series and in this way there is induced on the electrodea potential, with respect to a reference point, that is indicative ofthe relative position of the electrode and the elements. An apparatus ofthis kind is described in more detail in U.S. Patent 3,071,758 to P. C.F. Wolfendale.

An apparatus of this kind has exceptional properties. The inherentproperties of the electrical system are such that the ultimatesensitivity is much higher than is required for all normal metrologicalrequirements and is equal to or better than that of optical systems. Inpractice, with relatively simple constructions of the potentiometer asensitivity of a few microinches or better can be readily attained.

The present invention is concerned with improvements in the constructionof potentiometers for use in apparatus of this kind, the improvementsbeing directed to arrangerrient by which the effects due to mechanicalfeatures or inevitable imperfections of the potentiometers are reducedor. eliminated. v

More particularly one imperfection which I have noted is a residualperiodic variation which causes the response of the apparatus to departslightly from a regular law. This irregularity is caused if an edge ofthe pick-off electrode passes over a similarly shaped edge of one of theelements arranged in line.

It is a more detailed object of the invention to reduce the amplitude'ofsuch irregularities.

, Inaccordance with the present invention the edges of the pick-offelectrode structure which extend transverse to the direction of relativemovement, and of the transverse edges of the elements of the series arechosen to be different so that in effect the transition of an edge ofthe pick-off electrode structure over an edge of one of 3,348,133Patented Oct. 17, 1967 the elements in the line is made more gradual toreduce irregularities in the output characteristics.

Features and advantages of the invention will appear also from thefollowing description of embodiments thereof, given by way of example,in conjunction with the accompanying drawings, in which:

FIGURE 1 is a simplified diagram showing the general arrangement of aposition-response apparatus;

FIGURES 2 to 11 are diagrams showing mrangements of elements andelectrodes in accordance with the invention;

FIGURES 12 to 17 are diagrams showing possible arrangements of theelements;

FIGURE 18 is a diagrammatic side view of another form of elementarrangement.

FIGURE 19 is a diagram showing a disposition of a series of elements andthe interpolating electrodes, and FIGURE 20 is an exploded diagrammaticperspective view of an annular type series of elements and interpolatingelectrodes.

FIGURE 1 is an example of a simple position responsive or controlarrangement; this is an example only of a suitable arrangement andothers are to be found in U.S. Patent 3,071,758. The apparatus shown inFIGURE 1 includes a capacitative potentiometer indicated generally bythe broken line rectangle 40 and including a series of adjacentconductive elements 41, with insulating separators (not shown) betweenadjacent elements. As shown in FIGURE 1 the elements are arranged in astraight line, but this is not necessarily so, as will be apparenthereinafter.

Associated with the line of elements 41 is a pick-off or interpolatingelectrode system shown diagrammatically as an electrode 42; thiselectrode is spaced from and insulated from the elements 41; theelectrode system 42 can be moved along or with respect to the line ofelements 41, and in doing so will present a capacitance between it andthe adjacent one or ones of the elements.

Different potentials are established along the series of elements 41,the means for this purpose consisting conveniently of a tapped inductiveelement 43, connected to an alternating current source 44. In this waysuccessively increasing potentials are impressed upon the elements ofthe series, and hence, as the pick-off electrode moves along theelements of the series there will be induced on it a voltage indicativeof its position relative to the elements.

The voltage thus induced can be related to the voltage at a referencepoint; for example a reference potential can be obtained from areference voltage device 45 energized from source 44. In a very simplearrangement the reference point could be a tap on the inductive element43, but as it will usually be desired to vary the voltage of thereference point finely and with precision, it is preferable to use othermeans affording a fine control of voltage. A tapped inductive elementcan be used or preferably a sequence of such elements, in the mannerdescribed in the copending application referred to, so as to enable thereference point to be selected with successive scales or decades ofaccuracy. The means for varying the reference point is indicateddiagrammatically at 46, in FIGURE 1.

The alternating current potential produced on the pickoff electrode 42is fed, through amplifier 47, to one input of a voltage comparator 48,to the other input of which is fed a voltage from the reference point46.

Where the potentiometer is being used as part of an arrangement forindicating the position of a movable ele- For example, if the apparatusis being used in connection with a machine tool, the potentiometer mightbe connected to the bed of the machine, as indicated diagrammatically at49 in FIGURE 1, the moving electrode 42 being coupled to or associatedwith the tool slide 50 so as to move with it.

The relative position of the two parts 49 and 50 can be indicated by anindicating instrument 51 which is connected to the output of comparator48. This is a convenient method of indicating position, but is nothighly accurate as it depends upon the constancy of output of source 44.A far greater accuracy is attainable by taking advantage of the factthatthe circuit is a bridge circuit and, for any given position of electrode42 adjusting the reference voltage unitl the bridge is balanced, asindicated by the null reading of meter 51.

Where the potentiometer forms part of an arrangement for controlling theposition of the movable element, the arrangement is substantiallysimilar to that just described, but with the difference that thedeviation signal output from comparator 48 is used to control eitherdirectly or indirectly the operation of a reversible motor 52 whichserves to drive the movable element 50 and therefore also the electrode42 in a correcting sense; A null-seeking arrangement is used, so thatwhen the deviation signal falls If with such a construction theinterpolating electrode has the same length as one of the elements, itwill be seen that the two edges of the electrode will pass at the sametime over the transition points between adjacent elements. Thesetransitions involve small non-linearities in the relationship betweenthe voltage induced in the electrode and its relative position. It isvery advantageous for this relationship to be linear, and so thereduction or elimination of the irregularities is desirable. The presentinvention enables such irregularities to be reduced.

One way of achieving the desired transitional effect as the edges of theelectrode move over the edges of the adjacent elements is to have theelements defined by edges that are straight and parallel, and to modifythe shape of the electrode so that its surface is not bounded byparallel edges or if bounded by parallel edges the edges are at adifferent inclination, considered in the direction of relative movement,from those of the elements.

Examples of constructions of this kind are shown in FIGURES 2 to 9. Inthese figures the elements 60 are shown as being of equal extent alongthe line of the elements, that is, in the direction of relativemovement, and bounded by edges at right angles to that direction. It isconvenient, but not necessary, that this should be so. Also, in practicethe elements are usually stationary, because more connections to theelements are required; and the electrode moving. This arrangement willbe assumed here, though other arrangements are possible.

In FIGURES 2 to 11, the elements are indicated at 60, with interposedinsulation at 61. The elements can be flat, with their surfaces lying ina common plane, or cylindrical, with their outside surface again lyingon a common, cylindrical surface. The moving electrode 62 willaccordingly be plane or cylindrical.

Electrode 62 is shown as having a variety of shapes. Thus FIGURE 2 showsa pick-off electrode 62 which is rhombus or diamond-shaped with itsedges lying at an angle to the edges of elements 60 which are paralleland at right angles to the line of elements. In' FIGURE 3 a triangularelectrode is shown whilst in FIGURE 4 the electrode is a regularhexagon. With both these figures it will be seen that the movingelectrode does not extend across the full width of the surface of theelement. The

onal shapes. FIGURES 5 to 9 show various other useful shapes. In thearrangements of FIGURES 5 and 6, the pick-off electrode has respectivelya hexagonal and a regular octagonal shape, the octagonal shape being apresently preferred arrangement. In FIGURE 7 the pick-off electrode takean elongate rhomboid shape, with its ends parallel to the direction ofmovement. The circular and oval shapes which the pick-off electrodetakes respectively in FIGURES 8 and 9 are useful particularly where theoutput characteristics are non-linear.

Another variation of electrode is shown in FIGURES 10 and 11. It will beappreciated that one of the factors which controls the manner in whichthe pick-off electrode interpolates between the voltages applied tosuccessive electrodes is the extent of overlap in the linear direction,of the pick-off electrode and one of the elements. It may in some casesbe advantageous for the pick-off electrode to extend the direction ofrelative movement, over more than one element, and a shape of electrodesuitable for this purpose is shown in FIGURES 10 and 11. In FIG- URE 10the longitudinal edge of an electrode 62 is serrated at 63 and in FIGURE11 both edges are serrated in this way.

The use of linear and non-linear edges for the elements and theelectrode can be interchanged and FIGURES 12 to 16 of the drawings showarrangements of elements of the potentiometer which, in conjunction witha pick-of! electrode having a rectilinear edge or edges, will produce amore gradual transition from one element to the next. The elements areshown as applied to a linear arrangement, but can be adapted to anarrangement, suitable for responding to angular movements, in which theelements are formed as segments of an annulus.

FIGURE 12 shows elements the edges of which are parallel but inclined tothe length of the line of elements, the elements being separated byinsulation 61. In FIGURE '13 the edges of the elements 60 are defined bytwo straight portions arranged at an angle forming a chevron; in FIG-URE 14 the arrangement is similar to that of FIGURE 13 but with thepattern repeatedacross the width of the ele- I ments, whilst in FIGURE15 a combination of parts ar:

ranged both at right angles to the length of the elements and at anangle to them is provided. In FIGURE 16 the edges of the elements arecurved. In FIGURE 17, the elements 60 are of triangular shape, insulatedby spacers 61,

A form of the element suited to a circular construction is showninFIGURE 18, in which each element 60 takes the form of one turn of ahelix; for clarity, one such element is shown shaded. The insulation 61between elements includes a longitudinal part 65; with this arrangementit is preferred to use an electrode consisting of an incomplete annulus,or two segments of an incomplete an-. nulus, with the gap between theparts of the electrode lying over the. line of insulation 65. V

In FIGURES 10 and 11, the electrode occupies a length of more than oneelement, and other forms of this arrangement is shown in FIGURES 19 and20. The arrangement can be applied to both linear and angular type ele-'ments and involves the use of multiple electrodes so positioned that asthe electrodes move, transitions over the edges of different electrodesover the elements do not occur simultaneously with all the electrodes.In FIGURE 19 a series of elements 60 co-operate with three electrodeswhich move together, the electrodes being spacedby non-integralmultiples of the lengths of the elements, so that the electrodes do nothave the same relative position with respect to the adjacentelement. Inthis way non linearities occurring due to imperfections at thetransitions are averaged out. FIGURE 20 shows such an arrange ment forangular response with a series of elements cooperating with multipleelectrodes 70.

The various forms of potentiometer elements and electrodes describedafford the facility of varying the characteristic of variation effectedas the elements are moved relatively to the electrode. By such means itis possible to change considerably the way in which the desiredpotential is derived from the potentiometer, as the parts moverelatively. In addition, as will be apparent from my copendingapplication Ser. No. 140,874 filed September 1961 and issued as Patent3,278,716 on Nov. 22, 1966. Further variation can be obtained by choiceof the appropriate potentials applied to the linear elements of thepotentiometer.

I claim:

1. In a position responsive apparatus for deriving an output alternatingsignal having an amplitude which is a function of the relative positionsof two relatively movable members, a potentiometer including relativelymovable first and second electrode means and guide means, said firstelectrode means including a line of at least three similar electricallyseparate electrically conductive plates presenting respective firstelectrode surfaces aligned in at least one dimension and in edge to edgerelation, said second electrode means including a pick-ofl platepresenting a pick-off electrode surface spaced from said first electrodesurfaces, and said guide means defining a path for movement of saidpick-ofi electrode surface parallel to said first electrode surfaces,said surfaces having transverse edges which extend transverse to saidpath, at least one transverse edge of said pick-oif electrode surfacebeing disposed in such relation to said path as to pass over atransverse edge of said first electrode surfaces during movement alongsaid path, said apparatus also including an alternating supply forestablishing a reference potential, individual connection means forconnection between said supply and respective plates of said line forestablishing potentials on those plates which vary progressively alongsaid line with respect to said reference potential, whereby to establishan electric field directed along said path of movement which induces apotential on said pick-0E plate which is a function of its position withrespect to said line of plates, mechanical coupling means for couplingsaid electrode means respectively to said members, and output meanscoupled to said pick-off plate for deriving said output signal, whereinthe improvement comprises choosing the configuration of the transverseedges of said pick-off electrode surface to be dissimilar to theconfiguration of the transverse edges of each said first electrodesurface whereby to reduce irregularities in the output characteristicsof the apparatus.

2. Apparatus as claimed in claim 1 wherein said electrode plates arepolygonal, said line of plates being a rectilinear line.

3. Apparatus as claimed in claim 2 wherein said pickoif plate has adifferent number of sides to said aligned plates.

4. Apparatus as claimed in claim 3 wherein said pickoff plate forms aregular octagon, and said first electrode plates are rectangular.

References Cited UNITED STATES PATENTS 2,892,526 6/ 1959 Devaud 34020O X2,978,638 4/1961 Wing et -al 317-249 X 3,068,457 12/1962 Nevius 3l7-249X 3,142,795 7/ 1964 Greeley 323--93 3,146,394 8/1'964 'Frisch 323933,222,591 i12/1965 Mynall 323-74 X 3,223,919 '12/ 1965 Langm-an 323-43.5

JOHN F. COUCH, Primary Examiner.

A. D. PELLINEN, Assistant Examiner.

1. IN A POSITION RESPONSIVE APPRATUS FOR DERIVING AN OUTPUT ALTERNATINGSIGNAL HAVING AN AMPLITUDE WHICH IS A FUNCTION OF THE RELATIVE POSITIONSOF TWO RELATIVELY MOVABLE MEMBERS, A POTENTIOMETER INCLUDING RELATIVELYMOVABLE FIRST AND SECOND ELECTRODE MEANS AND GUIDE MEANS, SAID FIRSTELECTRODE MEANS INLCUDING A LINE OF AT LEAST THREE SIMILIAR ELECTRIALLYSEPARATE ELECTRICALLY CONDUCTIVE PLATES PRESENTING RESPECTIVE FIRSTELECTRODE SURFACES ALIGNED IN AT LEAST ONE DIMENSION AND IN EDGE TO EDGERELATION, SAID SECOND ELECTRODE MEANS INCLUDING A PICK-OFF PLATEPRESENTING A PICK-OFF ELECTRODE SURFACE SPACED FROM SAID FIRST ELECTRODESURFACES, AND SAID GUIDE MEANS DEFINING A PATH FOR MOVEMENT OF SAIDPICK-OFF ELECTRODE SURFACE PARALLEL TO SAID FIRST ELECTRODE SURFACES,SAID SURFACES HAVING TRANSVERSE EDGES WHICH EXTEND TRANSVERSE TO SAIDPATH, AT LEAST ONE TRANSVERSE EDGE OF SAID PICK-OFF ELECTRODE SURFACEBEING DISPOSED IN SUCH RELATION TO SAID PATH AS TO PASS OVER ATRANSVERSE EDGE OF SAID FIRST ELECTRODE SURFACE DURING MOVEMENT ALONGSAID PATH, SAID APPARATUS ALSO INCLUDING AN ALTERNATING SUPPLY FORESTABLISHING A REFERENCE POTENTIAL, INDIVIDUAL CONNECTION MEANS FORCONNECTION BETWEEN SAID SUPPLY AND RESPECTIVE PLATES OF SAID LINE FORESTABLISHING POTENTIALS ON THOSE PLATES WHICH VARY PROGRESSIVELY ALONGSAID LINE WITH RESPECT TO SAID REFERENCE POTENTIAL, WHEREBY TO ESTABLISHAN ELECTRIC FIELD DIRECTED ALONG SAID PATH OF MOVEMENT WHICH INDUCES APOTENTIAL ON SAID PICK-OFF PLATE WHICH IS A FUNCTION OF ITS POSITIONWITH RESPECT TO SAID LINE OF PLATES, MECHANICAL COUPLING MEANS FORCOUPLING SAID ELECTRODE MEANS RESPECTIVELY TO SAID MEMBERS, AND OUTPUTMEANS COUPLED TO SAID PICK-OFF PLATE FOR DERIVING SAID OUTPUT SIGNAL,WHEREIN THE IMPROVEMENT COMPRISES CHOOSING THE CONFIGURATION OF THETRANSVERSE EDGES OF SAID PICK-OFF ELECTRODE SURFACE TO BE DISSIMILAR TOTHE CONFIGURATION OF THE TRANSVERSE EDGES OF EACH SAID FIRST ELECTRODESURFACE WHEREBY TO REDUCE IRREGULARITIES IN THE OUTPUT CHARACTERISTICSOF THE APPARATUS.